These small crabs, scuttling in large 'herds' across the marsh at low tide, are among the most conspicuous invertebrates in Georgia salt marshes. Of the approximately 100 fiddler species scattered around the globe in mangroves, swamps and intertidal flats (http://en.wikipedia.org/wiki/Fiddler_crab), the Georgia coast has three common species: the mud fiddler in the lower salt marsh, the sand fiddler in the sandy upper marsh and beach lagoons, and the red-jointed fiddler in brackish tidal habitats such as black needlerush stands and coastal river marshes.
Georgia Outdoors has a terrific video about salt marsh fiddlers (and other crabs), that is well worth watching:
Recently another fiddler species, common in Florida mangrove stands, has been found on the Georgia coast. The large claw of male mangrove fiddlers is orange (see photo), in contrast to the pale claws of the endemic crabs.
A male mangrove fiddler crab caught in a salt marsh in Camden County, Georgia. The crab's large orange claw is aggressively pinching a finger. Image provided with permission by Dr. Clay Montague.